Purpose: Developing web applications rapidly
Django was created by a couple of nerds working for the newspaper industry, and they needed to solve the problem of having two practically identical sites (representing two newspapers owned by the same company) that had the ability to share content and generally be controlled from a central location. Thus Django, a flexible web framework that is different enough from Rails to be worth talking about.
Django is a batteries included framework that spends a lot of time trying to solve little things that are typically left to the gem community with Rails. The end result is a uniformly styled and extremely well documented web framework that can get you rolling pretty fast and is still easy enough to extend.
I've been using it for my new job and I've had very few complaints overall.
Purpose: My job
Summary: C and its descendants (C++/Java/C#/etc.) are the most popular programming languages in the world. (As co-inventor Dennis Ritchie supposedly said, "C is quirky, flawed, and an enormous success.") The web browser you're using, the OS, most of your applications, most embedded software, the software on your router, the software on your ISP's router, the software on your game console, etc., were all written in C (well, or in C++).
For my job, I write in straight C. No libraries, no frameworks, no C++, no nothin'. C is a programming language for Real Men (tm), which means that there's no memory management and you're free to crash your program in various horrific ways. The trade-off for this is speed and size, attributes which give C its staying power - though proper C is losing favor as an application development language to C++, C#, and the like, it has found a second life in embedded applications and other small devices. I also think there's a certain elegance to the syntax - it's a language from an era when you didn't have a lot of memory, so statements are terse and lack the cruft of some more modern languages (*cough*C++*cough*). And best of all, no right minded C programmer would use LongVariableNamesLikeThis.
There's no shortage of manuals and documentation for the various incarnations of C, but the best reference is still from the source: The C Programming Language, by Kernighan and Ritchie. This book is so ubiquitous and standard that it's known simply as K&R in the field.
Framework: ASP.NET MVC 2 (soon to jump up to 3)
Purpose: Web Development
ASP.NET MVC is Microsoft's answer to the slew of MVC frameworks already available for a variety of open source languages - Rails for Ruby, Django (is that actually a MVC framework?) for Python, and Zend, Code Igniter, and Kohana for PHP. It's a nice addition because, well, web forms suck for the web, and it follows the same basic overall design methodology as everyone else. It's now in its 3rd version, with a completely new view engine - Razor - which is actually pretty cool. I like where Microsoft is going with MVC.
There are a ton of free resources and tutorials available to get started. Like everything else, MVC is fairly simple to learn but difficult to master. It can be as complex as you want it to be.
Personally, I'm having some growing pains getting myself up to the next level/tier in my own skill development. Learning TDD, IoC, Domain Driven Design, etc. I understand the basics, but I'm still learning OOP in general and both C# and ASP.NET, so piling the more advanced (to me) stuff on top of it has been slow going.
Still, I like it more than PHP.
Purpose: Low level development
If you thought assembly language was low level, try Verilog or any of the other HDL languages on for size. Verilog is designed to describe how bits change every clock cycle. And not just one bit either, but potentially every bit available in the device that you are developing for.
This allows for massive parallelism - the sheer number of calculations per clock cycle can easily exceed both general purpose processors and DSPs.
It can also drive men insane.
I see square waves everywhere.
Purpose: Embeddable scripting
Lua is a neat little language. It is a dynamic, prototype-based language with relatively simple syntax (LL(1) ho!). There are primitive types (bools, numbers, strings, functions, nil), but the only structure for composition is the table, an associative array. Primitive types (except functions) are coerced to other primitive types as needed for operations. Functions are first class objects and it has closures.
Objects are created through special tables known as metatables, which define common operations and allow tables to take on characteristics of a class of objects, in effect allowing single inheritance.
The language is implemented in C and is designed to integrate easily with a host application. The API allows the host application to perform any operation the language can (and more). Lua can freely call functions provided by the host identically to native Lua functions and the host can create special object types that act as any other Lua object.
I mostly use it as a way to get scripting support into C, not as a standalone language, so I don't really know of any frameworks. I use a custom one to provide limited visibility of C++ classes to the scripts.
Oh and someone wrote a JIT compiler
Purpose: Video game tools/graphics
If you want to make AAA games on a console. You're probably going to end up working with C++, if you like it or not. With 512 megs of shared memory, multiple fiddly "special processing units", people clamoring over sending 64 players' worth of data over a network at an even pace and other wonderful things, performance down to the bit really does matter. Not to mention that the API (and compiler) is written for C++, so you're not getting away from it if you want to work on console games unless you're using XNA. C++ is the tacticool gun of programming languages. If you can do it, you can probably find a way to do it in C++, then you can probably find a way to hack it so that it only uses 10 bits at a time to do it.
Honestly, I don't spend the entirety of my time in this nether realm of pure data as I'm a tools programmer by trade. In a day I'll go between C++, C#, python and back again. Each language has its own sets of advantages and disadvantages, of the three I'd honestly say C# is the most "fun" to work with. C++ still wins out for me, just for being extremely versatile, while keeping performance high. I think the other thing I like about C++ is that the performance cost of anything is laid bare much more in the other languages I work with. Because you are tasked with moving around the bits other languages abstract away, I always feel the performance cost of code I write is much more impressed upon me when I write it in C++. When looking at performance in other langages, I often consider how it would effect performance had I tried the same trick, as underneath the hood the process is most likely similar.
Purpose: Web development
Although slightly outshone by the relatively new Ruby on Rails, PHP is still a solid choice for Web development. An engine is available for pretty much every web server (Apache and IIS being the major ones of course), it's easy to learn if you come from any kind of c type background and it offers some really neat features if you dig deep enough. Recent releases (5.3 I think) offer true namespacing to add to the plethora of object orientated features already present (if you like that kind of thing). One of the best things about PHP though is that, because it's so widely adopted there's literally tons of tutorials, documentation and samples out there to get you going.
Language: VB (classic)
Framework: Ha, I wish
Purpose: Legacy application development
Yeah I know. I use VB in my day job since I have to maintain a ton of applications written in it. It's slow as hell, the IDE sucks and I really have nothing good to say about it. For all its flaws .Net is a massive improvement on Microsoft's legacy development environments. I did manage to find a plugin for the VB IDE that allows tabbed documents, full screen editing and some other nifty features. I'll see if I can find it if anyone is interested.
Framework: jQuery (and jQuery mobile)
Purpose: Web application front end development
I also do C# and Android development, but can't really think of anything interesting to say about them right now.
Framework: .Net 4.0 Runtime
Purpose: HFT/Non-HFT systems
With Visual Studio 2010, F#, an ML-variant functional language, is now part of the .Net language family. It has full interop capabilities with any existing .Net assemblies and any other .Net languages are capable of loading .Net assemblies written in F# (with a couple of minor issues to watch out for). It's a full functional language and is best when you program it like a functional language and not ML with classes. There's some good resources out there on F#. I've done a couple of larger scale server applications with it and starting to move on to version 2.0 on a few of them. I also mix in C# when needed for things like COM-interop and certain client APIs.
Purpose: Large-scale data spelunking
Clojure is a neat little functional language that runs in the JVM. Very LISPy, with a heavy emphasis on macros. I mainly use it with Cascading/Hadoop to slam through the massive data sets and extract the various data of interest.
I also putter around with the CUDA/CULA stuff and data parallel Haskell when I have time.
Language: OCaml / C++ / Fortran
Framework: Lisp converted to Ocaml handed To grad students
Purpose: Combinatorial Optimization, Automated Planning, Robot Path finding, other research topics
OCaml is, like F#, an ML-variant with objects. It's particularly nice because it isn't terribly pedantic and lets you mix imperative programming with functional code wherever you feel it's expedient to do so. It's got a full object system which I've never extensively used, but I hear it's nice. You can run the code in an interactive interpreter, or you can compile native binaries which are relatively quick for a language which manages your memory for you.
The big drawback is that we don't have a concurrent garbage collector yet, so while we have threads, they don't behave the way you would want them to. You can work around it by doing any concurrency you'd like at the process level with pipes or something like MPI.
Purpose: Developing web applications
Ruby on Rails (RoR or often just called 'Rails') is a web application framework with a practical slant. While most frameworks present themselves as a sort of toolbox, Rails goes a step further by favoring convention over configuration. Instead of configuring how the tools interact with each other yourself, Rails infers what you mean to do from a few naming conventions in your class, method, table and path names. If it gets in the way, you can always define what name it should look for instead yourself.
Rails uses the model-view-controller (MVC) architectural pattern to separate the concerns in your code. On the controller side, it favors RESTful style url method coupling. On the model side, it provides an object oriented representation of your database tables. For the views, it provides a templating engine called ERB (I prefer HAML though).
One of the best things of Rails is the developer community. A lot of Rails developers blog about their experiences or post their problems on Stack Overflow. There also is a sort of package manager/repository for Ruby libraries called RubyGems that helps you install, update and resolve dependencies. For configuring what gems you use in your Rails project, you should use Bundler (which is baked into Rails 3). Most gems can be found on github for easy forking.
I can heartily recommend Rails to everyone looking for an easy to use web application framework. It's as easy as "sudo apt-get install rails && rails new ~/myproject".
Purpose: Manipulating your datas
SQL is ubiquitous and often taken for granted. Whether you're a Java or C or Access or PHP or what-have-you developer, you'll often be dealing with another language, being SQL. Some might have frameworks that abstract and/or obscure the SQL, but it's almost always there. The complexity required of your SQL can vary, and for a lot of projects it is relatively simple. Understanding SQL at a non-trivial level however will help you understand how computers work with large datasets, which will aid you in how you design and interact with your data even if you don't actually write any SQL directly.
Relational algebra and key theory is useful stuff for "thinking about it right" when it comes to schemas and queries. Also keep in mind that while SQL is a standard, every database system has a point where it diverges from the standard. When you start dealing with very complex queries or procedural code and triggers etc., you'll see very different syntax and often different approaches altogether due to vendor support of features available. For example, Microsoft SQL Server uses Transact-SQL (T-SQL), Oracle uses PL/SQL, and while both are the common system found in the business world and accomplishing the same objectives they are very different beasts to the developer. Methods and tricks for one are not always the best or feasible for the other, and you often rely on tricks to attain the performance demanded by the project.
In a rather different scope, web sites and services tend to use other systems, such as MySQL and PostgreSQL. The focus here is usually less on procedural code and more on efficient SQL-standard queries. The scale of the project might be trivially small where any design works to massive commerce sites that sell a hojillion products and track customer trends. Most people here will not be dealing with that, but many of us will have some sort of SQL database backend which we need to write queries for. Non-standard SQL is avoided as much as possible typically, in order to avoid vendor lock-in. This is the dangerous realm of SQL injection attacks which are one of the most common mistakes made by novice developers who need to use a database for persisting their data on their web site but don't have much experience or exposure. Sanitize your inputs and use parameterized queries!